Overcoming drug resistance to nucleoside analogs by tumor targeted active drug nanoformulations cytotoxic nucleoside analogs (NA) are important components of single-drug or multidrug chemotherapeutic regimens. However, drug resistance to therapeutic NA became constant clinical challenge in the treatment of tumors and viral infections. The principle mechanisms of resistance include deficiencies in drug transport and kinase-dependent activation of NA into 5'-triphosphates (NATP), an active drug form. Our central hypothesis is that direct delivery of NATP in the cytoplasm of cancer cells would be sufficient to bypass many mechanisms of drug resistance and efficiently eradicate drug-resistant tumors. This approach would also allow for decreasing toxic consequences of chemotherapy. We have specially designed nanocarriers for encapsulation of NATP, biodegradable nanogel cationic networks, which are capable to reversibly bind NATP, deliver and release the active drug inside cancer cells in tumor sites. In preliminary studies, NATP-nanogel formulations demonstrated significantly improved over NA therapeutic effect in many normal and drug-resistant cancer cells and animal models. Systemic circulation and tumor accumulation of nanoformulations will be optimized by modifying nanogel structure on nanoscale and decorating nanogel surface with multiple selected peptides having high affinity to overexpressed tumor EGF receptors or tumor lymphatic vessels. Nanogels degrade in tissues with the formation of non-toxic polymer conjugates. NATP-nanogel formulations retain their properties and can be stored in lyophilized form. This approach provides additional prospects for multidrug chemotherapy of drug-resistant tumors by administration of two NA molecules with different cellular targets. Systemic administration suggested for peptide-decorated nanogels would enhance drug accumulation in disseminated metastases and therapeutic efficacy against metastatic tumors. Our Specific Aim 1 is to develop efficient tumor-targeted nanogel carriers that are optimized for systemic delivery of NATP to tumors. Specific Aim 2 is to evaluate cytotoxic effect of drug nanoformulations in vitro in the collection of resistant to NA cancer cell lines. The collection of cell lines with specific mechanisms of resistance to NA will allow us to determine the most efficient combinations of vector, carrier and drug for elimination of drug-resistant cancer cells. In Specific Aim 3 we are going to achieve efficient therapy of drug-resistant tumors by selected nanoformulations in human breast cancer and lymphoma xenograft animal models and evaluate the feasibility of polychemotherapeutic approach using two NATP having separate cellular targets. Concisely stating, this approach develops a novel nanotechnology-based strategy of treating drug-resistant tumors.